1. Field of the Invention
The present invention relates to tyres for motor vehicles, particularly for use on snow-covered ground.
2. Description of the Related Art
As is known, a tyre in its most general form comprises a carcass structure, which includes a central crown portion and two axially opposed side walls terminating in a pair of beads for securing the tyre to the rim of a wheel, a belt structure associated coaxially with the carcass structure and a tread extending coaxially around the belt structure.
The tread comprises a relief pattern, generally formed by a plurality of transverse and longitudinal grooves which, in combination, give rise to a plurality of blocks distributed in different types of configurations, for example along a central area extending across the equatorial plane and in at least two shoulder areas extending in axially opposed positions on either side of the central area.
Of particular importance for the purposes of travel on a snow-covered surface is the presence, in the blocks, of suitable sipes, in other words a closely spaced set of notches or grooves oriented essentially transversely with respect to the direction of rolling, and having small dimensions in the circumferential direction, the function of these being to collect and retain the snow in an efficient way, since it has generally been shown to be convenient to exploit the friction of snow on snow.
One type of winter tyre marketed by the applicant has, in the tread, a pair of rectilinear circumferential grooves, arranged symmetrically with respect to the equatorial plane of the tyre and spaced apart by a distance essentially equal to half of the total width of the tread.
These circumferential grooves are combined with two sets of transverse grooves, each of which extends with a progressively increasing inclination from the outer edge of the tread to a point on the equatorial plane. The transverse grooves of the two sets converge symmetrically about the equatorial plane of the tyre in a predetermined preferred direction of rolling, and have their vertices interleaved with each other alternately along the perimetric extension of the tread on the equatorial plane.
In this tread, the configuration of the transverse grooves is such as to promote good characteristics of traction because of the mutual convergence of the grooves in the direction of rolling.
The circumferential rectilinear grooves, in turn, enable water to be removed efficiently from the footprint during travel on a wet surface, thus impeding the occurrence of the well-known and dangerous effect of aquaplaning.
The traction and the overall behaviour in travel on a snow-covered surface are improved by the provision of closely spaced sipes on the blocks generated by the intersection of the longitudinal grooves with the transverse ones and by suitable rectilinear recesses, associated with the transverse grooves, in the space delimited by the two circumferential grooves, each of the said rectilinear recesses extending in the circumferential direction for a short distance from the corresponding transverse groove.
Patent EP-773,116 held by the present applicant also discloses a tyre for snow-covered surfaces, provided with a tread band in which the circumferential grooves are formed by a sequence of portions oriented obliquely with respect to the circumferential direction of the tyre; these oblique portions converge symmetrically on the equatorial plane but in the opposite direction to the direction of convergence of the transverse grooves.
Each of the oblique portions of the circumferential grooves extends between two consecutive transverse grooves and delimits the opposing circumferential edges of a centre block in one case and a shoulder block in the other case.
In this solution, each of the centre and shoulder blocks has a corner projecting into the corresponding circumferential groove with respect to the opposite corner of the block immediately following it.
In this situation, the projecting corners of the centre blocks act as teeth which provide grip on snow in traction, while the projecting corners of the shoulder blocks produce this effect during braking.
In order to increase the grip on snow, this tyre also has a recess of trapezoidal shape extending partially between two adjacent blocks in the same centre row.
This recess passes through the transverse groove between the two centre blocks and has an inner circumferential edge and an outer circumferential edge which diverge from each other at angles preferably within the range from 3° to 15°, in the opposite direction to the direction of rolling.
However, the pattern of a tread suitable for gripping on snow, being of the block type, causes a certain degree of noise on dry roads because of the successive impacts of the transverse sides of the blocks on the ground.